The present invention relates to a stemming arrangement and stemming method for use in blast holes having one or more explosive charges therein.
In many applications of blasting, for example in mining, a number of blast holes are drilled and one or more explosive charges is placed at the end of the blast hole. Detonation of the explosive charges then ruptures the surrounding rock to enable it to be removed.
Stemming of blast holes involves plugging the blast holes in order to avoid reduction in the effectiveness of the blasting operation caused by the escape of blast gases. Stemming has previously been carried out by simply filling the blast holes with suitable material, such as angular particles, which then resists escape of the blast gases because of the tendency of the particle of stemming material to lock together by (so-called xe2x80x9cbridgingxe2x80x9d) to resist flow and because of the friction of the stemming material against the blast hole walls.
However, such existing stemming techniques suffer from the drawback that the stemming material can under certain circumstances still be ejected from the blast hole, for example, when the outward force of blast gases is so great that fluidising of the stemming material causes the stemming material to flow towards the mouth of the blast hole. This not only reduces the effectiveness of the blast, but the ejection of stemming material out of the blast hole can also be hazardous. In order to minimise this effect, the length of blast hole available and which can be effectively stemmed is sometimes insufficient to allow the use of the desired optimum size of explosive charge.
U.S. Pat. No. 4,754,705 discloses a mechanical stemming construction for blast holes in which a tapering wedge member is positioned in the blast hole outwardly of an explosive charge with its narrower end facing towards the mouth of the blast hole. When the wedge member is located in the blast hole, particulate stemming material is loaded into the blast hole outwardly of the wedge member. Detonation of the explosive charge in the blast hole then causes the wedge member to be driven towards the mouth of the blast hole which in turn drives the wedge member into the stemming material. The outward movement of the wedge member compacts the stemming material causing the particles thereof to xe2x80x9cbridgexe2x80x9d and forces the stemming material laterally against the walls of the blast hole.
However, this stemming construction suffers from the drawback that it is often unsuitable for use in wet blast holes because of the tendency of moisture in the blast hole to penetrate the stemming material used and thus reduce the tendency of the particles of stemming material to xe2x80x9cbridgexe2x80x9d effectively. The wedge member of this construction is also required to be constructed from rigid materials.
In addition, this type of prior art stemming construction is often unsuitable for many decking applications, i.e. in which a number of explosive charges are separated by stemming material in the same blast hole and are detonated according to a predetermined sequence. Effective stemming in decking applications is important since ineffective stemming may cause the detonation of one explosive charge to compress an adjacent charge in the same blast hole thus rendering that charge ineffective. The construction of U.S. Pat. No. 4,754,705 is less flexible in decking applications in which stemming between two explosive charges in a single blast hole is required, since the wedge member is only effective against detonation gases arriving from one side thereof. This makes the construction unsuitable for use in a decking arrangement in which an explosive charge facing the narrower end of the wedge member is to be detonated before a charge facing the wider end.
A further disadvantage of this prior art stemming construction is that it generally requires the provision of a stabilising rod to correctly orient the wedge member in the blast hole. A protective layer of stemming material is located outwardly of the explosive charge to minimise the risk of the wedge member being destroyed by the initial detonation, and the wedge member is then located outwardly of the protective layer and with the appropriate orientation by means of the stabilising rod. The stabilising rod is then removed while the remainder of the blast hole is filled with stemming material. This procedure is often very difficult, if not impossible, to correctly achieve in the case of non vertical blast holes or blast holes of narrow diameter. In addition, mis-orientation of the wedge member can often occur after detonation.
German Patent Application No 1287989 discloses an elastic spherical body which in use contains an air-water mixture under excess pressure. The body is located outwardly in a borehole of water-containing capsules, and is of a flexible nature having an unconfined diameter greater than that of the borehole. This prior art device is intended primarily to serve to assist in maintaining the water-containing capsules in place prior to detonation and is not intended to fulfil any stemming function after detonation. Accordingly, the extent to which this device can provide any effective post-detonation stemming is very limited.
U.S. Pat. No. 4,913,233 discloses an inflatable device for plugging a borehole to enable explosives to be located at a desired location in the borehole. The device is used to preserve an unfilled void in the borehole such that upon detonation, high pressure gases may flow and do work in the void, thereby reducing the total amount of explosive used and increasing the effective use of explosives. It is an essential requirement that this prior art device allows gas to flow to the void, as a result of which the device has insufficient strength to resist gas flow caused by detonation of an explosive charge.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.
According to an aspect of the present invention, there is provided a stemming arrangement for a blast hole having at least one explosive charge therein, the arrangement comprising:
at least one hollow member arranged in use in the blast hole outwardly of an associated explosive charge,
wherein detonation of the associated explosive charge deforms the or each hollow member so as to substantially block the bore of the blast hole at the location of said hollow member to substantially impede outward passage of material past said hollow member, and the or each hollow member is made of sufficiently strong material to remain substantially functionally intact after detonation of the associated explosive charge.
By providing a hollow member which deforms to substantially block the bore of the blast hole, this gives the advantage of in effect creating an impermeable membrane which substantially prevents the passage of material past the or each hollow member. By xe2x80x9cmaterialxe2x80x9d is meant gas and/or liquid and/or fluidised stemming material. As a result, the arrangement can therefore be effectively used in wet blast holes.
By xe2x80x9cfunctionally intactxe2x80x9d is meant that the hollow member remains sufficiently intact as to effect control of explosively induced forces during the rock breaking process caused by the detonation.
Preferably, the or each hollow member has a respective internal surface substantially enclosing a respective internal space.
It will be appreciated by persons skilled in the art that the term xe2x80x9csubstantially enclosexe2x80x9d encompasses one or more hollow members which do not entirely enclose the respective internal space, but allow air to pass from the interior of the hollow member to the exterior thereof on deformation of the hollow member as a result of detonation of the associated explosive charge.
The or each hollow member may be substantially liquid tight.
In a preferred embodiment, the or each hollow member is substantially spherical.
This has the advantage of making the stemming arrangement particularly suitable for decking applications because it is effective against detonation gases arriving from both sides thereof in the blast hole. By providing a substantially spherical member, this gives the further advantage of minimising any tendency of the detonation forces to rotate the hollow member about an axis transverse to the blast hole axis, which otherwise could result in failure of the stemming operation.
The or each hollow member may comprise first and second cooperating substantially hemispherical portions.
The or each hollow member is preferably at least partially filled with stemming material.
In a preferred embodiment, the or each hollow member is at least partially filled with a stemming material comprising gravel.
The or each hollow member preferably comprises plastics material.
This has the advantage of enabling the hollow member to be manufactured relatively inexpensively.
The plastics material may comprise polyethylene.
Alternatively, the plastics material may comprise PET.
The arrangement may further comprise a buffer layer of stemming material arranged between a said hollow member and the associated explosive charge.
This has the advantage of minimising damage to the hollow member as a result of the initial detonation of the associated explosive charge which may impede the operation of the arrangement to block the passage of detonation gases.
The arrangement preferably further comprises particulate stemming material arranged in the blast hole, at least part of the stemming material being arranged outwardly of the or each hollow member.
In a preferred embodiment the or each hollow member has a wall thickness between substantially 0.5% to 75% of the external diameter of said hollow member, and preferably between substantially 1% and 6% of the external diameter of said hollow member.
According to another aspect of the invention, there is provided a method of stemming a blast hole having at least one explosive charge therein, the method comprising:
locating at least one hollow member in the blast hole outwardly of an associated explosive charge,
wherein detonation of the associated explosive charge deforms the or each hollow member so as to substantially block the bore of the blast hole at the location of said hollow member and substantially impede outward passage of material past said member, and the or each said hollow member made of sufficiently strong material to remain substantially functionally intact after detonation of the associated explosive charge.
The method preferably further comprises the step of providing a buffer layer of stemming material between a said hollow member and the associated explosive charge.
The method preferably further comprises the step of locating stemming material in the blast hole such that at least part of the stemming material is arranged outwardly of the or each hollow member.